1. Field of the Invention
The present invention relates to a backlight inverter for a Liquid Crystal Display (LCD) such as a large screen LCD TV and a large screen LCD monitor, and more particularly to a one-lamp and one-transformer type backlight inverter which can precisely detect a lamp current by using electromagnetic induction and full-wave rectification at a second side of a main transformer in order to control the lamp current to be constant and stable, thereby maintaining luminous uniform.
2. Description of the Related Art
As LCD-TV and LCD monitor markets are growing gradually, more lamps are mounted on a backlight unit along followed by increase in size and length. Therefore, driving techniques for the lamps are also changing variously.
According to such trend, conventional LCD backlight inverters are also grouped into one type applicable to small screen LCDs having a screen size of about 17 inches and the other type applicable to large screen LCDs having a screen size exceeding 17 inches.
An inverter applied to a small screen LCD generates a first lamp voltage of about 500 Vrms to 800 Vrms, and a feedback node for detecting and stabilizing a lamp current is formed at a cold end of the lamp.
On the other hand, a backlight inverter applied to a large screen uses a long straight lamp or U-shaped lamp relatively longer than that of the conventional small screen LCD backlight inverter. In addition, regarding that such a long lamp also requires a second voltage of about 1 KVrms, there are still several technical problems to solve in order to drive the long lamp properly. One of the problems is how to detect a current flowing through the lamp and how to control the lamp in a constant current mode.
In particular, the backlight inverter for a large screen LCD uses two transformers to drive one lamp. Since hot and cold ends of the lamp are not clearly discriminated from each other, and current sensing at the cold end is not preferable unlike the small screen LCD inverter, the backlight inverter controls lamp current via another route. One of such conventional backlight inverters is shown in FIG. 1.
FIG. 1 is a block diagram illustrating a conventional backlight inverter for an LCD.
Referring to FIG. 1, a conventional backlight inverter for a large screen LCD includes first and second transformers 21 and 22 in order to actuate a U-shaped lamp 30 which needs a high first voltage of about 1 KVrms. Each of the first and second transformers 21 and 22, upon receiving a square wave signal from each of first and second drive units 11 and 12, steps up and converts it into an AC signal. Then, the first and second transformers 21 and 22 supply first and second AC driving currents AS1 and AS2 to both sides of the lamp 30. Here, the first and second AC driving currents are out of phase with each other.
In order to control the currents flowing through the lamp 30 steady, the backlight inverter also includes a current detector 40 with feedback nodes connected to the ground side of a second coil of the first and second transformers 21 and 22, respectively. The current detector 40 detects and feeds back a voltage induced by a current flowing through the second side of a corresponding transformer.
Based upon the currents detected by the current detector 40, the drive controller 50 controls the first and second drive units 11 and 12 to flow constant driving voltages through the lamp 30.
FIG. 2 is a diagram illustrating a driving voltage waveform of the backlight inverter shown in FIG. 1.
Referring to FIG. 2, first and second AC driving currents AS1 and AS2 supplied from the first and second transformers 21 and 22 to the lamp 30 are out of phase with each other. This out-of-phase status allows the driving currents to flow through the lamp 30.
In order to detect such driving currents, the current detector 40 includes a first resistor R1 connected to the ground side of the second coil of the first transformer 21 and a second resistor R2 connected to the ground side of the second coil of the second transformer 22.
The conventional backlight inverter for a large screen LCD as stated above needs two transformers to operate one U-shaped lamp 30, which disadvantageously raises cost. Such a disadvantage is an obstacle against miniaturization and low cost of products. Therefore, the conventional driving method type is not appropriate in the aspect of size and cost.
As an approach to overcome such a disadvantage, a new type has been researched and developed, which can drive two U-shaped lamps and four linear lamps. In the one-transformer and two-lamp type or one-transformer and four-lamp type, a considerably high voltage ranging from 750 V to 1,000 V is supplied to the lamps. Therefore, it is required to detect a current flowing through the lamps according to the turn ratio of first and second coils L21 and L22.